This invention relates to a marine engine exhaust system.
It is common to employ a vertical drive shaft in-line two-stroke cycle engine in the power head of an outboard marine propulsion system. With this type of engine, it is known to provide an exhaust flow path in which exhaust from the upper cylinders is collected and routed through an exhaust passage to a substantially vertical branch. Exhaust from the lower cylinders is collected and routed in a similar manner to a substantially vertical branch, which is in line with the branch from the upper cylinders and which receives exhaust therefrom. The lower branch thus forms a common exhaust passage which receives exhaust from all cylinders, which exhaust is then routed downwardly into an exhaust collection cavity provided in the upper end of the drive shaft housing.
By design, this arrangement provides effective exhaust tuning at high engine speeds to increase horsepower. However, operation at low speed is not satisfactory because certain exhaust ports effectively are supplied with a negative pulse, which detracts greatly from performance under low speed and idle conditions. Most notably, this design provides a rough idle and a decrease in low-end horsepower.
One solution to this problem has been to cut off the supply of fuel to certain cylinders at low engine speed. In a four cylinder engine, for example, the number three and four cylinders continue to receive fuel, while the fuel supply to the number one and two cylinders is cut off. This eliminates negative pulses at the exhaust ports of the number three and four cylinders, which provides smoother idle operation. The result, of course, is a substantial decrease in available horsepower at low speed operation.
It is an object of the present invention to solve the above problems, and to provide an engine having satisfactory operation at both high and low engine speeds by providing proper exhaust tuning. The invention is utilized in connection with an internal combustion engine including two or more cylinders, first and second exhaust passages and an exhaust discharge. Each exhaust passage receives exhaust from at least one of the cylinders, and the exhaust passages are arranged so as to be in communication with each other. In accordance with the invention, the improvement to such a structure comprises a third exhaust passage having an inlet in communication with one of the first or second exhaust passages, and valve means actuable between an open position and a closed position for selectively controlling the flow of exhaust through the third exhaust passage. The valve means in its open position allows flow of exhaust through the third exhaust passage and cuts off exhaust flow through the second passage. In its closed position, the valve means allows no flow of exhaust through the third passage and restores communication between the first and second passages, so that exhaust flowing through the first passage merges with exhaust flowing through the second passage prior to discharge. The third passage preferably has an outlet in communication with the exhaust discharge. The valve means is preferably disposed at or adjacent to the inlet to the third exhaust passage, and when open directs substantially all exhaust upstream thereof into the third passage and cuts off flow of such exhaust into the downstream passage. Actuator means is provided for moving the valve means between its open and closed positions. In one embodiment, the actuator means comprises a solenoid having a retractable and extendable plunger, which is operatively connected to a valve actuating arm for moving the valve between its open and closed positions. The solenoid is preferably interconnected with means outputting a signal proportional to engine speed. In this manner, the valve is opened at low engine speeds to provide proper exhaust tuning and improved operation of the engine, and closed at higher rpms.